Add identify overload with anonymousId param

[nwagu]

This is my attempt to close #233.

[jamesolrog]

I think the idea here is very interesting and might be worth re-addressing.

To expand on what is already here, there could also be a case where developers may wish to set the anonymous identifier without a formal user identifier.

For example, if a developer wishes to track users from an anonymous source (like an ad campaign) into a a non-authenticated experience (like an instant app), could it also be possible to modify the Analytics Configuration was to allow for an "initialAnonymousId" to be set?